Acid-sensing ion channel (ASIC) 4 predominantly localizes to an early endosome-related organelle upon heterologous expression.

Acid-sensing ion channels (ASICs) are voltage-independent proton-gated amiloride sensitive sodium channels, belonging to the DEG/ENaC gene family. Six different ASICs have been identified (ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, ASIC4) that are activated by a drop in extracellular pH, either as homo- or heteromers. An exception is ASIC4, which is not activated by protons as a homomer and which does not contribute to functional heteromeric ASICs. Insensitivity of ASIC4 to protons and its comparatively low sequence identity to other ASICs (45%) raises the question whether ASIC4 may have different functions than other ASICs. In this study, we therefore investigated the subcellular localization of ASIC4 in heterologous cell lines, which revealed a surprising accumulation of the channel in early endosome-related vacuoles. Moreover, we identified an unique amino-terminal motif as important for forward-trafficking from the ER/Golgi to the early endosome-related compartment. Collectively, our results show that heterologously expressed ASIC4 predominantly resides in an intracellular endosomal compartment.

Role for CD74 and CXCR4 in clathrin-dependent endocytosis of the cytokine MIF.

Macrophage migration inhibitory factor (MIF) is a pleiotropic cytokine that plays a role in innate and adaptive immunity. Depending on the cellular context and disease state, MIF signaling is mediated by its receptors CXCR2, CXCR4 and/or CD74. Although it is known that MIF is endocytosed, the exact mechanism has remained unknown. In exploring the mechanism of MIF endocytosis with biologically active Alexa(546)MIF, pathway-specific inhibitors (monodansylcadaverine, MDC; chlorpromazine, CPZ; dynasore; dominant-negative dynamin, bafilomycin, nocodazole) and receptor overexpression and blockade approaches, we identified a clathrin/dynamin-dependent endocytosis pathway as the main track for MIF internalization. MIF endocytosis was rapid and colocalization with both early and late endosomal vesicles in a microtubule- and acidification-dependent manner was observed. LDL endocytosis (which is clathrin-mediated) served as a control and was similarly inhibited by MDC or dynasore. When MIF endocytosis was compared to that of transferrin, acetylated LDL, and choleratoxin B (the latter internalized by a clathrin-independent pathway) by colocalization studies, the MIF internalization pathway clearly resembled that of LDL but also shared early trafficking with transferrin, whereas no colocalization with choleratoxin was noted. To identify the receptors involved in MIF endocytosis, we focused on CD74 and CXCR4 which form a heteromeric complex. Ectopic overexpression of CD74 in HEK293 and HeLa cells, which do not endogenously express CD74, led to a marked acceleration of MIF endocytosis while pharmacological blockade of CXCR4, which is endogenously expressed on these cells, with AMD3100 led to a 20% reduction of MIF endocytosis in HEK293-CD74 transfectants, whereas in untransfected cells, a blockade of 40% was observed. Of note, both CD74 and CXCR4 strongly colocalize with Alexa(546)MIF both on the plasma membrane and in endosomal compartments. Moreover, MIF-stimulated AKT signaling, which was previously shown to involve both CD74 and CXCR4, was reduced by endocytosis inhibitors, indicating that MIF signaling is at least in part due to endosomal signaling mechanisms. Thus, MIF uptake follows a rapid LDL-like, clathrin- and dynamin-dependent endocytosis pathway, which is dependent on the receptors CD74 and CXCR4 and leads to the initiation of endosomal signaling responses.

A functional heteromeric MIF receptor formed by CD74 and CXCR4.

MIF is a chemokine-like inflammatory mediator that triggers leukocyte recruitment by binding to CXCR2 and CXCR4. MIF also interacts with CD74/invariant chain, a single-pass membrane-receptor. We identified complexes between CD74 and CXCR2 with a role in leukocyte recruitment. It is unknown whether CD74 also binds to CXCR4. We demonstrate that CD74/CXCR4 complexes formed when CD74 was expressed with CXCR4 in HEK293 cells. Expression of CD74-variants lacking an ER-retention signal showed CD74/CXCR4 complexes at the cell surface. Importantly, endogenous CD74/CXCR4 complexes were isolated by co-immunoprecipitation from monocytes. Finally, MIF-stimulated CD74-dependent AKT activation was blocked by anti-CXCR4 and anti-CD74 antibodies and AMD3100, whereas CXCL12-stimulated AKT activation was not reduced by anti-CD74. Thus, CD74 forms functional complexes with CXCR4 that mediate MIF-specific signaling.